While polyolefins have achieved phenomenal economic success, they are in many aspects fragile polymers which require a variety of stabilizers or stabilizer mixtures to protect them from thermal, oxidative and actinic degradation. Such stabilizers include inter alia the phenolic antioxidants, the hindered amine stabilizers, the ultraviolet light absorbers, the organic phosphorous process stabilizers, alkaline metal salts of fatty acids and thiosynergists.
Although phenolic antioxidants have long been known to be very effective stabilizers for polyolefins and have enjoyed wide commercial success for that use, polyolefin compositions stabilized with phenolic antioxidants tend to discolor upon heating at elevated temperatures for prolonged periods or upon exposure to the combustion products of natural gas.
While the concomitant addition of organic phosphites to such polyolefin compositions mitigates the discoloration, it remains a serious practical problem. While the addition of phosphites is useful in preventing the discoloration of virgin polyolefins, which are not yet discolored, organic phosphites are not useful in reducing the color of polyolefins already discolored.
Likewise polyolefin compositions containing certain phenolic antioxidants and hindered amine light stabilizers tend to discolor upon storage for extended periods even at ambient temperatures.
As is described in U.S. Pat. No. 4,590,231, the concomitant use of a selected hydroxylamine, in addition to whatever other stabilizers are indicated, stabilized the polyolefin against degradation and discoloration upon exposure to heating at elevated temperatures, to the combustion products of natural gas, to gamma irradiation or to prolonged storage at ambient temperature.
U.S. Pat. No. 4,876,300 discloses that long chain N,N-dialkylhydroxylamines in the absence of any other stabilizers provide excellent process stabilization for polyolefin compositions both in regard to resistance to discoloration and to thermal degradation.
While these effects are certainly valuable and important, the polyolefins involved are virgin polymers which had not been previously processed, properly or improperly, nor had they been previously discolored by whatever method. Thus, these prior art references are silent as to whether hydroxylamines would be useful in the recycling of polyolefins, already discolored and/or degraded by improper processing at elevated temperatures or by prolonged aging, into products of sufficiently low color to make said products acceptable for practical use in the marketplace.
It is expected that increasing economic and environmental concerns about reducing the mountains of used waste polymers, including polyoolefins, will cause any method found feasible to recycle and reuse said used polymers to assume increasing importance in the future.